RFC2766 - Network Address Translation - Protocol Translation (NAT-PT)

　　Network Working Group G. Tsirtsis

　　Request for Comments: 2766 BT

　　Category: Standards Track P. Srisuresh

　　Campio Communications

　　February 2000

　　Network Address Translation - Protocol Translation (NAT-PT)

　　Status of this Memo

　　This document specifies an Internet standards track protocol for the

　　Internet community, and requests discussion and suggestions for

　　improvements. Please refer to the current edition of the "Internet

　　Official Protocol Standards" (STD 1) for the standardization state

　　and status of this protocol. Distribution of this memo is unlimited.

　　Copyright Notice

　　Copyright (C) The Internet Society (2000). All Rights Reserved.

　　Abstract

　　This document specifies an IPv4-to-IPv6 transition mechanism, in

　　addition to those already specified in [TRANS]. This solution

　　attempts to provide transparent routing, as defined in [NAT-TERM], to

　　end-nodes in V6 realm trying to communicate with end-nodes in V4

　　realm and vice versa. This is achieved using a combination of Network

　　Address Translation and Protocol Translation. The scheme described

　　does not mandate dual-stacks (i.e., IPv4 as well as V6 protocol

　　support) or special purpose routing requirements (such as requiring

　　tunneling support) on end nodes. This scheme is based on a

　　combination of address translation theme as described in [NAT-TERM]

　　and V6/V4 protocol translation theme as described in [SIIT].

　　Acknowledgements

　　Special thanks to Pedro Marques for reviewing an earlier version of

　　this memo. Also, many thanks to Alan O'Neill and Martin Tatham, as

　　the mechanism described in this document was initially developed

　　through discussions with them.

　　Table of Contents

　　1. Introduction.................................................. 2

　　2. Terminology................................................... 3

　　2.1 Network Address Translation (NAT)......................... 4

　　2.2 NAT-PT flavors............................................ 4

　　2.2.1 Traditional-NAT-PT................................... 4

　　2.2.2 Bi-directional-NAT-PT................................ 5

　　2.3 Protocol Translation (PT)................................. 5

　　2.4 Application Level Gateway (ALG)........................... 5

　　2.5 Requirements.............................................. 5

　　3. Traditional-NAT-PT operation (V6 to V4)....................... 6

　　3.1 NAT-PT Outgoing Sessions.................................. 6

　　3.2 NAPT-PT Outgoing Sessions................................. 7

　　4. Use of DNS-ALG for Address assignment......................... 8

　　4.1 V4 Address Assignment for Incoming Connections (V4 to V6). 9

　　4.2 V4 Address Assignment for Outgoing Connections (V6 to V4). 11

　　5. Protocol Translation Details.................................. 12

　　5.1 Translating IPv4 Headers to IPv6 Headers.................. 13

　　5.2 Translating IPv6 Headers to IPv4 Headers.................. 13

　　5.3 TCP/UDP/ICMP Checksum Update.............................. 13

　　6. FTP Application Level Gateway (FTP-ALG) Support............... 14

　　6.1 Payload modifications for V4 originated FTP sessions...... 15

　　6.2 Payload modifications for V6 originated FTP sessions...... 16

　　6.3 Header updates for FTP control packets.................... 16

　　7. NAT-PT Limitations and Future Work............................ 17

　　7.1 Topology Limitations...................................... 17

　　7.2 Protocol Translation Limitations.......................... 17

　　7.3 Impact of Address Translation............................. 18

　　7.4 Lack of End-to-End Security............................... 18

　　7.5 DNS Translation and DNSSEC................................ 18

　　8. Applicability Statement....................................... 18

　　9. Security Considerations....................................... 19

　　10. References................................................... 19

　　Authors' Addresses............................................... 20

　　Full Copyright Statement......................................... 21

　　1. Introduction

　　IPv6 is a new version of the IP protocol designed to modernize IPv4

　　which was designed in the 1970s. IPv6 has a number of advantages over

　　IPv4 that will allow for future Internet growth and will simplify IP

　　configuration and administration. IPv6 has a larger address space

　　than IPv4, an addressing model that promotes aggressive route

　　aggregation and a powerful autoconfiguration mechanism. In time, it

　　is expected that Internet growth and a need for a plug-and-play

　　solution will result in widespread adoption of IPv6.

　　There is expected to be a long transition period during which it will

　　be necessary for IPv4 and IPv6 nodes to coexist and communicate. A

　　strong, flexible set of IPv4-to-IPv6 transition and coexistence

　　mechanisms will be required during this transition period.

　　The SIIT proposal [SIIT] describes a protocol translation mechanism

　　that allows communication between IPv6-only and IPv4-only nodes via

　　protocol independent translation of IPv4 and IPv6 datagrams,

　　requiring no state information for the session. The SIIT proposal

　　assumes that V6 nodes are assigned a V4 address for communicating

　　with V4 nodes, and does not specify a mechanism for the assignment of

　　these addresses.

　　NAT-PT uses a pool of V4 addresses for assignment to V6 nodes on a

　　dynamic basis as sessions are initiated across V4-V6 boundaries. The

　　V4 addresses are assumed to be globally unique. NAT-PT with private

　　V4 addresses is outside the scope of this document and for further

　　study. NAT-PT binds addresses in V6 network with addresses in V4

　　network and vice versa to provide transparent routing [NAT-TERM] for

　　the datagrams traversing between address realms. This requires no

　　changes to end nodes and IP packet routing is completely transparent

　　[NAT-TERM] to end nodes. It does, however, require NAT-PT to track

　　the sessions it supports and mandates that inbound and outbound

　　datagrams pertaining to a session traverse the same NAT-PT router.

　　You will note that the topology restrictions on NAT-PT are the same

　　with those described for V4 NATs in [NAT-TERM]. Protocol translation

　　details specified in [SIIT] would be used to extend address

　　translation with protocol syntax/semantics translation. A detailed

　　applicability statement for NAT-PT may be found at the end of this

　　document in section 7.

　　By combining SIIT protocol translation with the dynamic address

　　translation capabilities of NAT and appropriate ALGs, NAT-PT provides

　　a complete solution that would allow a large number of commonly used

　　applications to interoperate between IPv6-only nodes and IPv4-only

　　A fundamental assumption for NAT-PT is only to be use when no other

　　native IPv6 or IPv6 over IPv4 tunneled means of communication is

　　possible. In other words the aim is to only use translation between

　　IPv6 only nodes and IPv4 only nodes, while translation between IPv6

　　only nodes and the IPv4 part of a dual stack node should be avoided

　　over other alternatives.

　　2. Terminology

　　The majority of terms used in this document are borrowed almost as is

　　from [NAT-TERM]. The following lists terms specific to this document.

　　2.1 Network Address Translation (NAT)

　　The term NAT in this document is very similar to the IPv4 NAT

　　described in [NAT-TERM], but is not identical. IPv4 NAT translates

　　one IPv4 address into another IPv4 address. In this document, NAT

　　refers to translation of an IPv4 address into an IPv6 address and

　　vice versa.

　　While the V4 NAT [NAT-TERM] provides routing between private V4 and

　　external V4 address realms, NAT in this document provides routing

　　between a V6 address realm and an external V4 address realm.

　　2.2 NAT-PT flavors

　　Just as there are various flavors identified with V4 NAT in [NAT-

　　TERM], the following NAT-PT variations may be identified in this

　　document.

　　2.2.1 Traditional NAT-PT

　　Traditional-NAT-PT would allow hosts within a V6 network to access

　　hosts in the V4 network. In a traditional-NAT-PT, sessions are uni-

　　directional, outbound from the V6 network. This is in contrast with

　　Bi-directional-NAT-PT, which permits sessions in both inbound and

　　outbound directions.

　　Just as with V4 traditional-NAT, there are two variations to

　　traditional-NAT-PT, namely Basic-NAT-PT and NAPT-PT.

　　With Basic-NAT-PT, a block of V4 addresses are set aside for

　　translating addresses of V6 hosts as they originate sessions to the

　　V4 hosts in external domain. For packets outbound from the V6 domain,

　　the source IP address and related fields such as IP, TCP, UDP and

　　ICMP header checksums are translated. For inbound packets, the

　　destination IP address and the checksums as listed above are

　　translated.

　　NAPT-PT extends the notion of translation one step further by also

　　translating transport identifier (e.g., TCP and UDP port numbers,

　　ICMP query identifiers). This allows the transport identifiers of a

　　number of V6 hosts to be multiplexed into the transport identifiers

　　of a single assigned V4 address. NAPT-PT allows a set of V6 hosts to

　　share a single V4 address. Note that NAPT-PT can be combined with

　　Basic-NAT-PT so that a pool of external addresses are used in

　　conjunction with port translation.

　　For packets outbound from the V6 network, NAPT-PT would translate the

　　source IP address, source transport identifier and related fields

　　such as IP, TCP, UDP and ICMP header checksums. Transport identifier

　　can be one of TCP/UDP port or ICMP query ID. For inbound packets, the

　　destination IP address, destination transport identifier and the IP

　　and transport header checksums are translated.

　　2.2.2 Bi-Directional-NAT-PT

　　With Bi-directional-NAT-PT, sessions can be initiated from hosts in

　　V4 network as well as the V6 network. V6 network addresses are bound

　　to V4 addresses, statically or dynamically as connections are

　　established in either direction. The name space (i.e., their Fully

　　Qualified Domain Names) between hosts in V4 and V6 networks is

　　assumed to be end-to-end unique. Hosts in V4 realm access V6-realm

　　hosts by using DNS for address resolution. A DNS-ALG [DNS-ALG] must

　　be employed in conjunction with Bi-Directional-NAT-PT to facilitate

　　name to address mapping. Specifically, the DNS-ALG must be capable

　　of translating V6 addresses in DNS Queries and responses into their

　　V4-address bindings, and vice versa, as DNS packets traverse between

　　V6 and V4 realms.

　　2.3 Protocol Translation (PT)

　　PT in this document refers to the translation of an IPv4 packet into

　　a semantically equivalent IPv6 packet and vice versa. Protocol

　　translation details are described in [SIIT].

　　2.4 Application Level Gateway (ALG)

　　Application Level Gateway (ALG) [NAT-TERM] is an application specific

　　agent that allows a V6 node to communicate with a V4 node and vice

　　versa. Some applications carry network addresses in payloads. NAT-PT

　　is application unaware and does not snoop the payload. ALG could work

　　in conjunction with NAT-PT to provide support for many such

　　applications.

　　2.5 Requirements

　　The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,

　　SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this

　　document, are to be interpreted as described in [KEYWORDS].

　　3. Traditional-NAT-PT Operation (V6 to V4)

　　NAT-PT offers a straight forward solution based on transparent

　　routing [NAT-TERM] and address/protocol translation, allowing a large

　　number of applications in V6 and V4 realms to inter-operate without

　　requiring any changes to these applications.

　　In the following paragraphs we describe the operation of

　　traditional-NAT-PT and the way that connections can be initiated from

　　a host in IPv6 domain to a host in IPv4 domain through a

　　traditional-NAT-PT

　　3.1 Basic-NAT-PT Operation

　　[IPv6-B]-+

　　| +==============+

　　[IPv6-A]-+-[NAT-PT]---------| IPv4 network |--[IPv4-C]

　　| +==============+

　　(pool of v4 addresses)

　　Figure 1: IPv6 to IPv4 communication

　　Node IPv6-A has an IPv6 address -> FEDC:BA98::7654:3210

　　Node IPv6-B has an IPv6 address -> FEDC:BA98::7654:3211

　　Node IPv4-C has an IPv4 address -> 132.146.243.30

　　NAT-PT has a pool of addresses including the IPv4 subnet

　　120.130.26/24

　　The V4 addresses in the address pool could be allocated one-to-one to

　　the V6 addresses of the V6 end nodes in which case one needs as many

　　V4 addresses as V6 end points. In this document we assume that the V6

　　network has less V4 addresses than V6 end nodes and thus dynamic

　　address allocation is required for at least some of them.

　　Say the IPv6 Node A wants to communicate with the IPv4 Node C. Node

　　A creates a packet with:

　　Source Address, SA=FEDC:BA98::7654:3210 and Destination

　　Address, DA = PREFIX::132.146.243.30

　　NOTE: The prefix PREFIX::/96 is advertised in the stub domain by the

　　NAT-PT, and packets addressed to this PREFIX will be routed to the

　　NAT-PT. The pre-configured PREFIX only needs to be routable within

　　the IPv6 stub domain and as such it can be any routable prefix that

　　the network administrator chooses.

　　The packet is routed via the NAT-PT gateway, where it is translated

　　to IPv4.

　　If the outgoing packet is not a session initialisation packet, the

　　NAT-PT SHOULD already have stored some state about the related

　　session, including assigned IPv4 address and other parameters for the

　　translation. If this state does not exist, the packet SHOULD be

　　silently discarded.

　　If the packet is a session initialisation packet, the NAT-PT locally

　　allocates an address (e.g: 120.130.26.10) from its pool of

　　addresses and the packet is translated to IPv4. The translation

　　parameters are cached for the duration of the session and the IPv6 to

　　IPv4 mapping is retained by NAT-PT.

　　The resulting IPv4 packet has SA=120.130.26.10 and DA=132.146.243.30.

　　Any returning traffic will be recognised as belonging to the same

　　session by NAT-PT. NAT-PT will use the state information to translate

　　the packet, and the resulting addresses will be

　　SA=PREFIX::132.146.243.30, DA=FEDC:BA98::7654:3210. Note that this

　　packet can now be routed inside the IPv6-only stub network as normal.

　　3.2 NAPT-PT Operation

　　NAPT-PT, which stands for "Network Address Port Translation +

　　Protocol Translation", would allow V6 nodes to communicate with the

　　V4 nodes transparently using a single V4 address. The TCP/UDP ports

　　of the V6 nodes are translated into TCP/UDP ports of the registered

　　V4 address.

　　While NAT-PT support is limited to TCP, UDP and other port

　　multiplexing type of applications, NAPT-PT solves a problem that is

　　inherent with NAT-PT. That is, NAT-PT would fall flat when the pool

　　of V4 addresses assigned for translation purposes is exhausted. Once

　　the address pool is exhausted, newer V6 nodes cannot establish

　　sessions with the outside world anymore. NAPT-PT, on the other hand,

　　will allow for a maximum of 63K TCP and 63K UDP sessions per IPv4

　　address before having no TCP and UDP ports left to assign.

　　To modify the example sited in figure 1, we could have NAPT-PT on the

　　border router (instead of NAT-PT) and all V6 addresses could be

　　mapped to a single v4 address 120.130.26.10.

　　IPv6 Node A would establish a TCP session with the IPv4 Node C as

　　follows:

　　Node A creates a packet with:

　　Source Address, SA=FEDC:BA98::7654:3210 , source TCP port = 3017 and

　　Destination Address, DA = PREFIX::132.146.243.30, destination TCP

　　port = 23.

　　When the packet reaches the NAPT-PT box, NAPT-PT would assign one of

　　the TCP ports from the assigned V4 address to translate the tuple of

　　(Source Address, Source TCP port) as follows:

　　SA=120.130.26.10, source TCP port = 1025 and

　　DA=132.146.243.30, destination TCP port = 23.

　　The returning traffic from 132.146.243.30, TCP port 23 will be

　　recognised as belonging to the same session and will be translated

　　back to V6 as follows:

　　SA = PREFIX::132.146.243.30, source TCP port = 23;

　　DA = FEDC:BA98::7654:3210 , destination TCP port = 3017

　　Inbound NAPT-PT sessions are restricted to one server per service,

　　assigned via static TCP/UDP port mapping. For example, the Node

　　[IPv6-A] in figure 1 may be the only HTTP server (port 80) in the V6

　　domain. Node [IPv4-C] sends a packet:

　　SA=132.146.243.30, source TCP port = 1025 and

　　DA=120.130.26.10, destination TCP port = 80

　　NAPT-PT will translate this packet to:

　　SA=PREFIX::132.146.243.30, source TCP port = 1025

　　DA=FEDC:BA98::7654:3210, destination TCP port = 80

　　In the above example, note that all sessions which reach NAPT-PT with

　　a destination port of 80 will be redirected to the same node [IPv6-

　　A].

　　4. Use of DNS-ALG for Address Assignment

　　An IPv4 address is assigned by NAT-PT to a V6 node when NAT-PT

　　identifies the start of session, inbound or outbound. Identification

　　of the start of a new inbound session is performed differently than

　　for outbound sessions. However, the same V4 address pool is used for

　　assignment to V6 nodes, irrespective of whether a session is

　　initiated outbound from a V6 node or initiated inbound from a V4

　　node.

　　Policies determining what type of sessions are allowed and in which

　　direction and from/to which nodes is out of the scope of this

　　document.

　　IPv4 name to address mappings are held in the DNS with "A" records.

　　IPv6 name to address mappings are at the moment held in the DNS with

　　"AAAA" records. "A6" records have also been defined but at the time

　　of writing they are neither fully standardized nor deployed.

　　In any case, the DNS-ALG's principle of operation described in this

　　section is the same with either "AAAA" or "A6" records. The only

　　difference is that a name resolution using "A6" records may require

　　more than one query - reply pairs. The DNS-ALG SHOULD, in that case,

　　track all the replies in the transaction before translating an "A6"

　　record to an "A" record.

　　One of the aims of NAT-PT design is to only use translation when

　　there is no other means of communication, such as native IPv6 or some

　　form of tunneling. For the following discussion NAT-PT, in addition

　　to the IPv4 connectivity that it has it may also have a native IPv6

　　and/or a tunneled IPv6 connection.

　　4.1 V4 Address assignment for incoming connections (V4 to V6)

　　[DNS]--+

　　| [DNS]------[DNS]-------[DNS]

　　[IPv6-B]-+ | |

　　| +==============+ |

　　[IPv6-A]-+----[NAT-PT]------| IPv4 network |--[IPv4-C]

　　| +==============+

　　(pool of v4 addresses)

　　Figure 2: IPv4 to IPv6 communication

　　Node IPv6-A has an IPv6 address -> FEDC:BA98::7654:3210

　　Node IPv6-B has an IPv6 address -> FEDC:BA98::7654:3211

　　Node IPv4-C has an IPv4 address -> 132.146.243.30

　　NAT-PT has a pool of addresses including the IPv4 subnet

　　120.130.26/24

　　In figure 2 above, when Node C's name resolver sends a name look up

　　request for Node A, the lookup query is directed to the DNS server on

　　the V6 network. Considering that NAT-PT is residing on the border

　　router between V4 and V6 networks, this request datagram would

　　traverse through the NAT-PT router. The DNS-ALG on the NAT-PT device

　　would modify DNS Queries for A records going into the V6 domain as

　　follows: (Note that a TCP/UDP DNS packet is recognised by the fact

　　that its source or destination port number is 53)

　　a) For Node Name to Node Address Query requests: Change the Query

　　type from "A" to "AAAA" or "A6".

　　b) For Node address to Node name query requests: Replace the

　　string "IN-ADDR.ARPA" with the string "IP6.INT". Replace the

　　V4 address octets (in reverse order) preceding the string "IN-

　　ADDR.ARPA" with the corresponding V6 address (if there exists a

　　map) octets in reverse order.

　　In the opposite direction, when a DNS response traverses from the DNS

　　server on the V6 network to the V4 node, the DNS-ALG once again

　　intercepts the DNS packet and would:

　　a) Translate DNS responses for "AAAA" or "A6" records into "A"

　　records, (only translate "A6" records when the name has

　　completely been resolved)

　　b) Replace the V6 address resolved by the V6 DNS with the V4

　　address internally assigned by the NAT-PT router.

　　If a V4 address is not previously assigned to this V6 node, NAT-PT

　　would assign one at this time. As an example say IPv4-C attempts to

　　initialise a session with node IPv6-A by making a name lookup ("A"

　　record) for Node-A . The name query goes to the local DNS and from

　　there it is propagated to the DNS server of the IPv6 network. The

　　DNS-ALG intercepts and translates the "A" query to "AAAA" or "A6"

　　query and then forwards it to the DNS server in the IPv6 network

　　which replies as follows: (The example uses AAAA records for

　　convenience)

　　Node-A AAAA FEDC:BA98::7654:3210,

　　this is returned by the DNS server and gets intercepted and

　　translated by the DNS-ALG to:

　　Node-A A 120.130.26.1

　　The DNS-ALG also holds the mapping between FEDC:BA98::7654:3210 and

　　120.130.26.1 in NAT-PT. The "A" record is then returned to Node-C.

　　Node-C can now initiate a session as follows:

　　SA=132.146.243.30, source TCP port = 1025 and

　　DA=120.130.26.1, destination TCP port = 80

　　the packet will be routed to NAT-PT, which since it already holds a

　　mapping between FEDC:BA98::7654:3210 and 120.130.26.1 can translate

　　the packet to:

　　SA=PREFIX::132.146.243.30, source TCP port = 1025

　　DA=FEDC:BA98::7654:3210, destination TCP port = 80

　　the communication can now proceed as normal.

　　The TTL values on all DNS resource records (RRs) passing through

　　NAT-PT SHOULD be set to 0 so that DNS servers/clients do not cache

　　temporarily assigned RRs. Note, however, that due to some buggy DNS

　　client implementations a value of 1 might in some cases work better.

　　The TTL values should be left unchanged for statically mapped

　　addresses.

　　Address mappings for incoming sessions, as described above, are

　　subject to denial of service attacks since one can make multiple

　　queries for nodes residing in the V6 network causing the DNS-ALG to

　　map all V4 addresses in NAT-PT and thus block legitimate incoming

　　sessions. Thus, address mappings for incoming sessions should time

　　out to minimise the effect of denial of service attacks.

　　Additionally, one IPv4 address (using NAPT-PT, see 3.2) could be

　　reserved for outgoing sessions only to minimise the effect of such

　　attacks to outgoing sessions.

　　4.2 V4 Address assignment for outgoing connections (V6 to V4)

　　V6 nodes learn the address of V4 nodes from the DNS server in the V4

　　domain or from the DNS server internal to the V6 network. We

　　recommend that DNS servers internal to V6 domains maintain a mapping

　　of names to IPv6 addresses for internal nodes and possibly cache

　　mappings for some external nodes. In the case where the DNS server in

　　the v6 domain contains the mapping for external V4 nodes, the DNS

　　queries will not cross the V6 domain and that would obviate the need

　　for DNS-ALG intervention. Otherwise, the queries will cross the V6

　　domain and are subject to DNS-ALG intervention. We recommend

　　external DNS servers in the V4 domain cache name mapping for external

　　nodes (i.e., V4 nodes) only. Zone transfers across IPv4 - IPv6

　　boundaries are strongly discouraged.

　　In the case of NAPT-PT, a TCP/UDP source port is assigned from the

　　registered V4 address upon detection of each new outbound session.

　　We saw that a V6 node that needs to communicate with a V4 node needs

　　to use a specific prefix (PREFIX::/96) in front of the IPv4 address

　　of the V4 node. The above technique allows the use of this PREFIX

　　without any configuration in the nodes.

　　To create another example from Figure 2 say Node-A wants to set up a

　　session with Node-C. For this Node-A starts by making a name look-up

　　("AAAA" or "A6" record) for Node-C.

　　Since Node-C may have IPv6 and/or IPv4 addresses, the DNS-ALG on the

　　NAT-PT device forwards the original AAAA/A6 query to the external DNS

　　system unchanged, as well as an A query for the same node. If an

　　AAAA/A6 record exists for the destination, this will be returned to

　　NAT-PT which will forward it, also unchanged, to the originating

　　host.

　　If there is an A record for Node-C the reply also returns to the

　　NAT-PT. The DNS-ALG then, translates the reply adding the appropriate

　　PREFIX and forwards it to the originating device with any IPv6

　　addresses that might have learned. So, if the reply is

　　NodeC A 132.146.243.30, it is translated to

　　NodeC AAAA PREFIX::132.146.243.30 or to

　　NodeC A6 PREFIX::132.146.243.30

　　Now Node A can use this address like any other IPv6 address and the

　　V6 DNS server can even cache it as long as the PREFIX does not

　　change.

　　An issue here is how the V6 DNS server in the V6 stub domain talks to

　　the V4 domain outside the V6 stub domain. Remember that there are no

　　dual stack nodes here. The external V4 DNS server needs to point to a

　　V4 address, part of the V4 pool of addresses, available to NAT-PT.

　　NAT-PT keeps a one-to-one mapping between this V4 address and the V6

　　address of the internal V6 DNS server. In the other direction, the V6

　　DNS server points to a V6 address formed by the IPv4 address of the

　　external V4 DNS servers and the prefix (PREFIX::/96) that indicates

　　non IPv6 nodes. This mechanism can easily be extended to accommodate

　　secondary DNS servers.

　　Note that the scheme described in this section impacts DNSSEC. See

　　section 7.5 of this document for details.

　　5. Protocol Translation Details

　　The IPv4 and ICMPv4 headers are similar to their V6 counterparts but

　　a number of field are either missing, have different meaning or

　　different length. NAT-PT SHOULD translate all IP/ICMP headers from v4

　　to v6 and vice versa in order to make end-to-end IPv6 to IPv4

　　communication possible. Due to the address translation function and

　　possible port multiplexing, NAT-PT SHOULD also make appropriate

　　adjustments to the upper layer protocol (TCP/UDP) headers. A separate

　　section on FTP-ALG describes the changes FTP-ALG would make to FTP

　　payload as an FTP packet traverses from V4 to V6 realm or vice versa.

　　Protocol Translation details are described in [SIIT], but there are

　　some modifications required to SIIT because of the fact that NAT-PT

　　also performs Network Address Translation.

　　5.1 Translating IPv4 headers to IPv6 headers

　　This is done exactly the same as in SIIT apart from the following

　　fields:

　　Source Address:

　　The low-order 32 bits is the IPv4 source address. The high-

　　order 96 bits is the designated PREFIX for all v4

　　communications. Addresses using this PREFIX will be routed

　　to the NAT-PT gateway (PREFIX::/96)

　　Destination Address:

　　NAT-PT retains a mapping between the IPv4 destination

　　address and the IPv6 address of the destination node. The

　　IPv4 destination address is replaced by the IPv6 address

　　retained in that mapping.

　　5.2 Translating IPv6 headers to IPv4 headers

　　This is done exactly the same as in SIIT apart from the Source

　　Address which should be determined as follows:

　　Source Address:

　　The NAT-PT retains a mapping between the IPv6 source address

　　and an IPv4 address from the pool of IPv4 addresses

　　available. The IPv6 source address is replaced by the IPv4

　　address retained in that mapping.

　　Destination Address:

　　IPv6 packets that are translated have a destination address

　　of the form PREFIX::IPv4/96. Thus the low-order 32 bits of

　　the IPv6 destination address is copied to the IPv4

　　destination address.

　　5.3 TCP/UDP/ICMP Checksum Update

　　NAT-PT retains mapping between IPv6 address and an IPv4 address from

　　the pool of IPv4 addresses available. This mapping is used in the

　　translation of packets that go through NAT-PT.

　　The following sub-sections describe TCP/UDP/ICMP checksum update

　　procedure in NAT-PT, as packets are translated from V4 to V6 and vice

　　versa.

　　5.3.1 TCP/UDP/ICMP Checksum Update from IPv4 to IPv6

　　UDP checksums, when set to a non-zero value, and TCP checksum SHOULD

　　be recalculated to reflect the address change from v4 to v6. The

　　incremental checksum adjustment algorithm may be borrowed from [NAT].

　　In the case of NAPT-PT, TCP/UDP checksum should be adjusted to

　　account for the address and TCP/UDP port changes, going from V4 to V6

　　address.

　　When the checksum of a V4 UDP packet is set to zero, NAT-PT MUST

　　evaluate the checksum in its entirety for the V6-translated UDP

　　packet. If a V4 UDP packet with a checksum of zero arrives in

　　fragments, NAT-PT MUST await all the fragments until they can be

　　assembled into a single non-fragmented packet and evaluate the

　　checksum prior to forwarding the translated V6 UDP packet.

　　ICMPv6, unlike ICMPv4, uses a pseudo-header, just like UDP and TCP

　　during checksum computation. As a result, when the ICMPv6 header

　　checksum is computed [SIIT], the checksum needs to be adjusted to

　　account for the additional pseudo-header. Note, there may also be

　　adjustments required to the checksum due to changes in the source and

　　destination addresses (and changes in TCP/UDP/ICMP identifiers in the

　　case of NAPT-PT) of the payload carried within ICMP.

　　5.3.2 TCP/UDP/ICMP Checksum Update from IPv6 to IPv4

　　TCP and UDP checksums SHOULD be recalculated to reflect the address

　　change from v6 to v4. The incremental checksum adjustment algorithm

　　may be borrowed from [NAT]. In the case of NAPT-PT, TCP/UDP checksums

　　should be adjusted to account for the address and TCP/UDP port

　　changes, going from V6 to V4 addresses. For UDP packets, optionally,

　　the checksum may simply be changed to zero.

　　The checksum calculation for a V4 ICMP header needs to be derived

　　from the V6 ICMP header by running the checksum adjustment algorithm

　　[NAT] to remove the V6 pseudo header from the computation. Note, the

　　adjustment must additionally take into account changes to the

　　checksum as a result of updates to the source and destination

　　addresses (and transport ports in the case of NAPT-PT) made to the

　　payload carried within ICMP.

　　6. FTP Application Level Gateway (FTP-ALG) Support

　　Because an FTP control session carries, in its payload, the IP

　　address and TCP port information for the data session, an FTP-ALG is

　　required to provide application level transparency for this popular

　　Internet application.

　　In the FTP application running on a legacy V4 node, arguments to the

　　FTP PORT command and arguments in PASV response(successful) include

　　an IP V4 address and a TCP port, both represented in ASCII as

　　h1,h2,h3,h4,p1,p2. However, [FTP-IPV6] suggests EPRT and EPSV command

　　extensions to FTP, with an intent to eventually retire the use of

　　PORT and PASV commands. These extensions may be used on a V4 or V6

　　node. FTP-ALG, facilitating transparent FTP between V4 and V6 nodes,

　　works as follows.

　　6.1 Payload modifications for V4 originated FTP sessions

　　A V4 host may or may not have the EPRT and EPSV command extensions

　　implemented in its FTP application. If a V4 host originates the FTP

　　session and uses PORT or PASV command, the FTP-ALG will translate

　　these commands into EPRT and EPSV commands respectively prior to

　　forwarding to the V6 node. Likewise, EPSV response from V6 nodes will

　　be translated into PASV response prior to forwarding to V4 nodes.

　　The format of EPRT and EPSV commands and EPSV response may be

　　specified as follows[FTP-IPV6].

　　EPRT<space><d><net-prt><d><net-addr><d><tcp-port><d>

　　EPSV<space><net-prt>

　　(or)

　　EPSV<space>ALL

　　Format of EPSV response(Positive): 229 <text indicating

　　extended passive mode> (<d><d><d><tcp-port><d>)

　　PORT command from a V4 node is translated into EPRT command, by

　　setting the protocol <net-prt> field to AF #2 (IPV6) and translating

　　the V4 host Address (represented as h1,h2,h3,h4) into its NAT-PT

　　assigned V6 address in string notation, as defined in [V6ADDR] in the

　　<net-addr>field. TCP port represented by p1,p2 in PORT command must

　　be specified as a decimal <tcp-port> in the EPRT command. Further,

　　<tcp-port>translation may also be required in the case of NAPT-PT.

　　PASV command from a V4 node is be translated into a EPSV command with

　　the <net-prt> argument set to AF #2. EPSV response from a V6 node is

　　translated into PASV response prior to forwarding to the target V4

　　host.

　　If a V4 host originated the FTP session and was using EPRT and EPSV

　　commands, the FTP-ALG will simply translate the parameters to these

　　commands, without altering the commands themselves. The protocol

　　Number <net-prt> field will be translated from AF #1 to AF #2.

　　<net-addr>will be translated from the V4 address in ASCII to its

　　NAT-PT assigned V6 address in string notation as defined in [V6ADDR].

　　<tcp-port>argument in EPSV response requires translation only in the

　　case of NAPT-PT.

　　6.2 Payload modifications for V6 originated FTP sessions

　　If a V6 host originates the FTP session, however, the FTP-ALG has two

　　approaches to pursue. In the first approach, the FTP-ALG will leave

　　the command strings "EPRT" and "EPSV" unaltered and simply translate

　　the <net-prt>, <net-addr> and <tcp-port> arguments from V6 to its

　　NAT-PT (or NAPT-PT) assigned V4 information. <tcp-port> is translated

　　only in the case of NAPT-PT. Same goes for EPSV response from V4

　　node. This is the approach we recommend to ensure forward support for

　　RFC2428. However, with this approach, the V4 hosts are mandated to

　　have their FTP application upgraded to support EPRT and EPSV

　　extensions to allow access to V4 and V6 hosts, alike.

　　In the second approach, the FTP-ALG will translate the command

　　strings "EPRT" and "EPSV" and their parameters from the V6 node into

　　their equivalent NAT-PT assigned V4 node info and attach to "PORT"

　　and "PASV" commands prior to forwarding to V4 node. Likewise, PASV

　　response from V4 nodes is translated into EPSV response prior to

　　forwarding to the target V6 nodes. However, the FTP-ALG would be

　　unable to translate the command "EPSV<space>ALL" issued by V6 nodes.

　　In such a case, the V4 host, which receives the command, may return

　　an error code indicating unsupported function. This error response

　　may cause many RFC2428compliant FTP applications to simply fail,

　　because EPSV support is mandated by RFC2428. The benefit of this

　　approach, however, is that is does not impose any FTP upgrade

　　requirements on V4 hosts.

　　6.3 Header updates for FTP control packets

　　All the payload translations considered in the previous sections are

　　based on ASCII encoded data. As a result, these translations may

　　result in a change in the size of packet.

　　If the new size is the same as the previous, only the TCP checksum

　　needs adjustment as a result of the payload translation. If the new

　　size is different from the previous, TCP sequence numbers should also

　　be changed to reflect the change in the length of the FTP control

　　session payload. The IP packet length field in the V4 header or the

　　IP payload length field in the V6 header should also be changed to

　　reflect the new payload size. A table is used by the FTP-ALG to

　　correct the TCP sequence and acknowledgement numbers in the TCP

　　header for control packets in both directions.

　　The table entries should have the source address, source data port,

　　destination address and destination data port for V4 and V6 portions

　　of the session, sequence number delta for outbound control packets

　　and sequence number delta for inbound control packets.

　　The sequence number for an outbound control packet is increased by

　　the outbound sequence number delta, and the acknowledgement number

　　for the same outbound packet is decreased by the inbound sequence

　　number delta. Likewise, the sequence number for an inbound packet is

　　increased by the inbound sequence number delta and the

　　acknowledgement number for the same inbound packet is decreased by

　　the outbound sequence number delta.

　　7. NAT-PT Limitations and Future Work

　　All limitations associated to NAT [NAT-TERM] are also associated to

　　NAT-PT. Here are the most important of them in detail, as well as

　　some unique to NAT-PT.

　　7.1 Topology limitations

　　There are limitations to using the NAT-PT translation method. It is

　　mandatory that all requests and responses pertaining to a session be

　　routed via the same NAT-PT router. One way to guarantee this would be

　　to have NAT-PT based on a border router that is unique to a stub

　　domain, where all IP packets are either originated from the domain or

　　destined to the domain. This is a generic problem with NAT and it is

　　fully described in [NAT-TERM].

　　Note, this limitation does not apply to packets originating from or

　　directed to dual-stack nodes that do not require packet translation.

　　This is because in a dual-stack set-up, IPv4 addresses implied in a

　　V6 address can be identified from the address format PREFIX::x.y.z.w

　　and a dual-stack router can accordingly route a packet between v4 and

　　dual-stack nodes without tracking state information.

　　This should also not affect IPv6 to IPv6 communication and in fact

　　only actually use translation when no other means of communication is

　　possible. For example NAT-PT may also have a native IPv6 connection

　　and/or some kind of tunneled IPv6 connection. Both of the above

　　connections should be preferred over translation when possible. The

　　above makes sure that NAT-PT is a tool only to be used to assist

　　transition to native IPv6 to IPv6 communication.

　　7.2 Protocol Translation Limitations

　　A number of IPv4 fields have changed meaning in IPv6 and translation

　　is not straightforward. For example, the option headers semantics and

　　syntax have changed significantly in IPv6. Details of IPv4 to IPv6

　　Protocol Translation can be found in [SIIT].

　　7.3 Impact of Address Translation

　　Since NAT-PT performs address translation, applications that carry

　　the IP address in the higher layers will not work. In this case

　　Application Layer Gateways (ALG) need to be incorporated to provide

　　support for those applications. This is a generic problem with NAT

　　and it is fully described in [NAT-TERM].

　　7.4 Lack of end-to-end security

　　One of the most important limitations of the NAT-PT proposal is the

　　fact that end-to-end network layer security is not possible. Also

　　transport and application layer security may not be possible for

　　applications that carry IP addresses to the application layer. This

　　is an inherent limitation of the Network Address Translation

　　function.

　　Independent of NAT-PT, end-to-end IPSecsecurity is not possible

　　across different address realms. The two end-nodes that seek IPSec

　　network level security must both support one of IPv4 or IPv6.

　　7.5 DNS Translation and DNSSEC

　　The scheme described in section 4.2 involves translation of DNS

　　messages. It is clear that this scheme can not be deployed in

　　combination with secure DNS. I.e., an authoritative DNS name server

　　in the V6 domain cannot sign replies to queries that originate from

　　the V4 world. As a result, an V4 end-node that demands DNS replies

　　to be signed will reject replies that have been tampered with by

　　NAT-PT.

　　The good news, however, is that only servers in V6 domain that need

　　to be accessible from the V4 world pay the price for the above

　　limitation, as V4 end-nodes may not access V6 servers due to DNS

　　replies not being signed.

　　Also note that zone transfers between DNS-SEC servers within the same

　　V6 network are not impacted.

　　Clearly, with DNS SEC deployment in DNS servers and end-host

　　resolvers, the scheme suggested in this document would not work.

　　8. Applicability Statement

　　NAT-PT can be a valuable transition tool at the border of a stub

　　network that has been deployed as an IPv6 only network when it is

　　connected to an Internet that is either V4-only or a combination of

　　V4 and V6.

　　NAT-PT, in its simplest form, without the support of DNS-ALG,

　　provides one way connectivity between an IPv6 stub domain and the

　　IPv4 world meaning that only sessions initialised by IPv6 nodes

　　internal to the IPv6 stub domain can be translated, while sessions

　　initiated by IPv4 nodes are dropped. This makes NAT-PT a useful

　　tool to IPv6 only stub networks that need to be able to maintain

　　connectivity with the IPv4 world without the need to deploy servers

　　visible to the IPv4 world.

　　NAT-PT combined with a DNS-ALG provides bi-directional connectivity

　　between the IPv6 stub domain and the IPv4 world allowing sessions to

　　be initialised by IPv4 nodes outside the IPv6 stub domain. This

　　makes NAT-PT useful for IPv6 only stub networks that need to deploy

　　servers visible to the IPv4 world.

　　Some applications count on a certain degree of address stability for

　　their operation. Dynamic address reuse by NAT-PT might not be

　　agreeable for these applications. For hosts running such address

　　critical applications, NAT-PT may be configured to provide static

　　address mapping between the host's V6 address and a specific V4

　　address. This will ensure that address related changes by NAT-PT do

　　not become a significant source of operational failure.

　　9. Security Considerations

　　Section 7.4 of this document states that end-to-end network and

　　transport layer security are not possible when a session is

　　intercepted by a NAT-PT. Also application layer security may not be

　　possible for applications that carry IP addresses in the application

　　layer.

　　Section 7.5 of this document states that the DNS-ALG can not be

　　deployed in combination with secure DNS.

　　Finally, all of the security considerations described in [NAT-TERM]

　　are applicable to this document as well.

　　10. REFERENCES

　　[DNS-ALG] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A.

　　Heffernan, "DNS extensions to Network Address Translators

　　(DNS_ALG)", RFC2694, September 1999.

　　[DNSSEC] Eastlake, D., "Domain Name System Security Extensions",

　　RFC2065, March 1999.

　　[FTP-IPV6] Allman, M., Ostermann, S. and C. Metz, "FTP Extensions for

　　IPv6 and NATs", RFC2428, September 1998.

　　[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate

　　Requirement Levels", BCP 14, RFC2119, March 1997.

　　[NAT] Egevang, K. and P. Francis, "The IP Network Address

　　Translator (NAT)", RFC1631, May 1994.

　　[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address

　　Translator (NAT) Terminology and Considerations", RFC

　　2663, August 1999.

　　[SIIT] Nordmark, E., "Stateless IP/ICMP Translator (SIIT)", RFC

　　2765, February 2000.

　　[TRANS] Gilligan, R. and E. Nordmark, "Transition Mechanisms for

　　IPv6 Hosts and Routers", RFC1933, April 1996.

　　[V6ADDR] Hinden, R. and S. Deering, "IP Version 6 Addressing

　　Architecture", RFC2373, July 1998.

　　Authors' Addresses

　　George Tsirtsis

　　Internet Futures

　　B29 Room 129

　　BT Adastral Park

　　IPSWICH IP5 3RE

　　England

　　Phone: +44 181 8260073

　　Fax: +44 181 8260073

　　EMail: george.tsirtsis@bt.com

　　EMail (alternative): gtsirt@hotmail.com

　　Pyda Srisuresh

　　630 Alder Drive

　　Milpitas, CA 95035

　　U.S.A.

　　Phone: (408) 519-3849

　　EMail: srisuresh@yahoo.com

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